KR20050104381A - Method and system for power efficient transmission of scalable video over wireless networks - Google Patents

Abstract

A method and system for reducing power consumption in a wireless network by adjusting the transmit energy for each bit at the physical layer and the retry limit at the MAC layer. The method includes creating a look-up table containing optimal pairs of Nlim,Et for a plurality of different sets of transmission properties, wherein Nlim is a retry limit and Etis a tr ansmit energy per bit; determing a set of transmission properties for a sequence of scalable video to be transmitted over the wireless network; accessing the look-up table to obtain the optimal pair of Nlim,Et, corresponding to the set of determined transmission properties; and transmitting the sequence of scalable video over the wireless network using the accessed optimal pair of Nlim,Et.

Description

METHOD AND SYSTEM FOR POWER EFFICIENT TRANSMISSION OF SCALABLE VIDEO OVER WIRELESS NETWORKS

TECHNICAL FIELD The present invention relates generally to wireless networks, and more particularly to methods and systems for power efficient transmission of scalable video over a wireless network (a wireless network comprising a portable multimedia device).

Because of the high throughput provided by wireless local area networks (WLANs), real-time video communications over WLANs are becoming possible. Possible applications include video communications on portable devices, portable video servers, and the like. Usually these types of WLAN devices rely on batteries for operation. Batteries have a limited lifetime and frequent recharging is undesirable. Power management is becoming more important for the completion of video transmissions that require high bandwidth and high transmit power.

1 shows the effect of the maximum retry limit N _lim on the overall transmit power;

FIG. 2 illustrates a Fine-Granular-Scalable (GFS) scalable video transmission system.

3 shows a PSNR for a simple video sequence;

4 illustrates transmission for different retry limits;

5A-5C show, for large P _L = 1%, the required transmit energy per bit needed for a given PSNR (E _t ), the average number of transmissions needed to successfully transmit one packet, and retransmission, A diagram showing transmit energy per bit for successfully transmitting one bit,

6A-6C show, for small P _L = 0.01%, the required transmit energy per bit needed for a given PSNR (E _t ), the average number of transmissions required to successfully transmit one packet, and retransmission, A diagram showing transmit energy per bit for successfully transmitting one bit,

7 shows power consumption versus distance,

8 is a flowchart according to an embodiment of the present invention;

9 is a transmission system according to an embodiment of the present invention;

10 is a computer system implementing the power manager of the present invention.

The drawings are only schematic representations and are not intended to show specific parameters of the invention. The drawings are intended to illustrate only typical aspects of the invention and therefore should not be considered as limiting the scope of the invention.

The present invention contemplates a situation in which scalable video data is transmitted over a WLAN and retransmission is employed as an error control technique. One object of the present invention is to maintain a constant video quality at the receiver side while minimizing the overall transmit power, or to optimize the video quality given the upside down fixed transmit power resources.

In the present invention, in order to minimize the transmission power while maintaining constant video quality at the receiver side, retransmission techniques in the physical layer and in the medium access control (MAC) layer are considered. In particular, the present invention reduces power consumption by adjusting the transmit energy for each bit in the physical layer and the retry limit in the MAC layer.

In general, the present invention provides a method for power efficient transmission of scalable video over a wireless network, the method comprising a lookup table comprising an optimal pair (N _lim , E _t ) for a plurality of different sets of transmission characteristics. N _lim is a retry limit and E _t is transmission energy per bit, determining a set of transmission characteristics for a scalable video sequence to be transmitted over a wireless network, and corresponding to the determined set of transmission characteristics. optimal pair using the step of accessing a look-up table to obtain the (N _lim, E _t) and the accessed optimal pair (N _lim, E _t) a step of transmitting a sequence of scalable video over a wireless network Include.

The present invention also provides a system for power efficient transmission of scalable video over a wireless network, which includes an optimal pair (N _lim , E _t ) for a number of different sets of transmission characteristics, where N _lim is The retry limit and E _t are the lookup table, which is the transmit energy per bit, and the set of transmit characteristics for the scalable video sequence to be transmitted over the wireless network, and the optimal pair (N _lim , E corresponding to the determined set of transmit characteristics). a system for accessing the lookup table to obtain _t ) and a system for transmitting a sequence of scalable video over the wireless network using the optimal pair (N _lim , E _t ) accessed.

The invention further provides a program product stored on a recordable medium that provides for power efficient transmission of scalable video over a wireless network, the program product having a set of transmission characteristics for a scalable video sequence to be transmitted over the wireless network. program codes and the optimal pair for a plurality of different transmission characteristics set for determining the (N _lim, E _t) at the best pair corresponding to the transmission characteristic set determined by accessing a look-up table containing (N _lim, E _t) Program code to obtain N _lim is the retry limit and E _t is the transmit energy per bit, and the scalable video sequence is transmitted over the wireless network using the optimal pair (N _lim , E _t ) accessed. .

These and other features of the present invention will be more readily understood from the following detailed description of various aspects of the invention in conjunction with the accompanying drawings.

The present invention describes a method and system for reducing transmission power consumption in scalable video communications over a wireless network (eg, WLAN). This is accomplished by selecting the maximum number of retransmissions based on quality and delay requirements. Thus, the transmitter SNR is adjusted to maintain constant end-to-end video quality. For different retry limits, and therefore for different transmitter SNRs, different power consumption is taken. The present invention finds and uses a transmission power level that minimizes overall energy for power efficient scalable video transmission. Using the present invention, scalable video over a wireless LAN by adjusting the retransmission limit and the transmit power level given a basic channel condition (SNR) that may be affected by noise, interference, and distance between the transmitter and receiver. Power efficient transmission of is achieved.

Referring now to FIG. 1 (ad), the effect of the maximum retry limit N _lim on the overall transmit power is shown. As shown, as the retry limit increases, in order to maintain the same end-to-end video quality, (1) the video stream can tolerate a higher packet loss rate, and (2) the transmission energy per bit is higher. (3) the number of retransmissions increases or remains the same, and (4) the overall transmit power varies.

In general, the larger the retry limit in the MAC layer, the more retransmitters the transmitter can have. Thus, the number of transmissions including the first transmission and subsequent retransmission (s) is an increasing function of the retry limit N _lim as shown in FIG. On the other hand, as the number of transmissions increases, the error control performance is enhanced, so that the packet loss rate can be higher before any retransmission, in order to maintain a given quality at the receiver side. That is, the stream can tolerate more errors caused by transmission. The relationship between the packet loss rate and the retry limit is shown in FIG. 1 (b). The transmit energy per packet is a decreasing function of the packet loss rate. Thus, it is a decreasing function of the retry limit N _lim , as shown in FIG. 1 (c). Combining Figures 1 (a) and 1 (c), the overall power as a function of transmit energy and number of transmissions has an optimal point N ^* _lim that minimizes power consumption and thus extends battery life.

As an illustrative example, a Fine-Granular-Scalable (GFS) coded video stream will be transmitted. At the MAC layer, the retry limit can be adapted to video quality requirements and basic channel conditions. At the physical layer, the energy for transmitting the bits can be adjusted. In the subsequent analysis, the bit stream is transmitted over an additive white Gaussian noise (AWGN) channel and the theoretical impact of the transmission energy on the retry limit and the overall transmit power is analyzed. Then, a numerical analysis is provided to select the optimal operating point that minimizes overall power consumption.

Analytical model

The system 10 contemplated by the present invention is shown in FIG. In system 10, the video stream is compressed by FGS encoder 12, modulated using differential phase shift keying, and transmitted over basic additive white Gaussian (AWGN) channel 14. The retry limit at the MAC layer 16, and the transmit energy at the physical layer 18, are tailored to video quality requirements and basic channel conditions. No channel encoder is used above the MAC layer.

The distortion caused by the FGS encoder 12 will first be described. Parameters and power consumption that cause a given distortion at the receiver side by adjusting the MAC layer 16 and the physical layer 18 are described in the following paragraphs.

FGS  Distortion Model of Video Encoder

FGS encoding provides a smooth quality degradation to adapt to changing network conditions. The FGS encoder 12 includes a base layer encoder 20 and an enhancement layer encoder 22. The base layer is compressed for the base layer encoder 20 using the motion compensation encoding method, and the enhancement layer encoder 22 is based on the fine granule coding method. In this description, it is assumed that all base layer bits are received without any error. Enhancement layer data is organized into packets and transmitted over an unreliable channel.

FGS  Of encoder PNSR Rate  Performance

FGS encoder 12 provides a nearly linear relationship between the enhancement layer bit rate and the peak signal to noise ratio (PSNR) as shown in FIG. 3 for a simple video sequence. This linear function can be expressed as

Where R _s is the encoded source bit rate and PSNR is the corresponding video PSNR. In Figure 3, the parameters are derived by the least mean square error method. For a simple video sequence encoded at a frame rate of 30 fps and a quantization step size of 10, the parameter values are shown in Table 1. It can be seen from FIG. 3 that there is a good match between the measurement data and the linear model.

Average at receiver if there is packet loss PSNR

In the preceding paragraph, it was explained that the reconstructed video quality measured in PSNR is the first order function of the encoded bit stream when no error occurs during transmission. The PSNR of the decoded video sequence in case of transmission error will be described in the case where a video sequence of frame rate of fr fps is given, each encoded frame consisting of N _el packets of base layer data and enhancement layer data. If one packet error occurs, it is assumed that all subsequent enhancement layer packets corresponding to the same video frame are discarded. I When the packet of the first correctly received for one frame, corresponding data rates in the receiver (R _i) is

to be. Where M is the packet size and R _bl is the data rate for the base layer. The parameters used for the numerical analysis in this disclosure are listed in Table 1. At the receiver, the video PSNR is

And the average PSNR at the receiver is

Where p _i is the probability that the first i packet is successfully received.

Define the data maintained at the receiver as valid data and the amount of valid data for one second as the effective data rate R _el , where R _el is calculated as follows:

Accordingly, you get

Thus, as long as a data rate of R _el can be obtained, one can expect that the receiver can roughly reach the corresponding PSNR.

If the residual packet loss rate after retransmission is p _L ,

Combining equations (2), (5) and (7), R _el can be expressed as follows.

In certain algorithms where N _el and M are fixed, R _el is determined by p _L. Therefore, the average PSNR at the receiving side is determined by the residual packet error rate p _L. In the following paragraphs, we will describe how p _L relates to the retry limit at the MAC layer, the transmit SNR at the physical layer, and power consumption.

Transmission model

The retry limit N _lim at the physical layer and how the transmitting SNR will control p _L at the receiving side, i.e. PSNR, will be described. The information bit stream is composed of packets, each containing M information bits. If it is detected that there is an error in the received packet, a packet error occurs (even one bit error can cause a packet error). The probability that a packet is an error, ie p _po , depends on the received signal-to-noise ratio per bit. The physical layer will first be described, where the bit error rate is determined by the channel characteristics and the transmit energy E _t applied to each bit. Next, we will discuss how retransmission reduces errors at the receiving side and how retransmission causes extra energy consumption using multiple transmissions for one video packet.

Packet error Rate p _p0

For simplicity, assume that the channel is an AWGN channel, DPSK is used for modulation, and E _t is the transmit energy per bit. The received energy Eb per bit is proportional to the path gain between two mobiles, depending on h, i.e. the distance between the two mobiles. That is, Eb = hE _t . Where h is given by

Where c is a constant and d is the distance between two stations. In this example, α = 3.6. The value of c is chosen such that when two mobiles are 100m apart, the received SNR per bit is 2 dB to 16 dB. Bit Error Rate (BER)

Where N ₀ is the noise power spectral density.

Packet errors occur even when there is only one bit error. For M bit packets, the packet error rate is

to be.

Residual Packet Loss Rate p _L

In a wireless LAN, retransmission is used as an error control technique. Only if all N _lim +1 transmissions are in error, the packet will not pass through the channel successfully. Therefore, the remaining packet error rate is a probability that the packet is an error after N _lim +1 transmission and is expressed as follows.

As shown in Fig. 4, when the retry limit is larger, more packets can be transmitted correctly.

Average number of transmissions N _tr

As shown in Fig. 4, each video packet is transmitted until it is successfully transmitted or until the retry limit is reached. The probability that a video packet is successfully transmitted on the nth attempt is p _p0 ^{n -1} (1-p _p0 ), and the probability that the transmission of the video packet is not successfully transmitted and reaches the retry limit is 1 Overall, the average number of transmissions for a video packet is

to be.

Whole transmission energy

From equation (12), for one video packet that has been successfully transmitted or discarded due to a limitation on the MAC layer retry, the average energy used is

to be.

For video sequences of frame rate of fr fps and for each frame containing N _el packets in the enhancement layer, the power consumption by the enhancement layer data is

to be. Thus, the optimization problem of the present invention can be formulated as Min P _all subject to delay and bandwidth constraints.

In this example, it is assumed that there is a sufficiently high bandwidth. The upper limit of the retry limit is set to satisfy the delay constraint, and the optimization problem can be specified as follows.

Numerical results

In this paragraph, the performance of the method of the present invention is investigated. First, performance under different quality requirements is considered when the distance between the transmitter and the receiver is fixed. Then, the quality requirements are the same, but the receiver is trained to move. The parameter values used in the simulations are summarized in Table 1. Here, a simple video sequence is encoded at a frame rate of 30 fps and the transmission data rate is 2.84 Mbps, which corresponds to a PSNR of 35 dB if no error occurs. The base layer data rate is 0.67 Mbps and the reconstructed PSNR from the base layer is 30.29 dB. Enhancement layer data is packetized into nine packets, each containing 1000 bytes. At the physical layer, the received signal to noise ratio is selected from 2 dB to 16 dB when the two mobiles are 10 meters apart. The maximum retry limit is set to N _upper = 20 to ensure that one packet can be received within the delay constraint.

In the figures describing performance (ie, figures 5a-5c, 6a-6c), power consumption is normalized by a scaling factor.

That is, the values shown in the drawings

to be.

Of different requirements PSNR Minimize for

In this section, we analyze how the optimal point depends on the requirements of the PSNR when the distance is fixed at d = 10. Different PSNR requirements at the receiving side will be considered. Two different p _L are used to simulate different quality requirements. The results are shown in FIGS. 5A-C and 6A-C.

As described in equations (14, 15 and 17) for a given video quality corresponding to p _L = 1%, the transmit energy per bit (E _t ) for a given PSNR, which is needed to successfully transmit one packet. The transmission energy per bit for successfully transmitting one bit, including the average number of transmissions and retransmission, is described in Figs. 5A-5C, respectively. As shown in FIG. 5A, as the retry limit increases, the same video quality can be obtained with lower energy E _t per bit. In addition, as shown in FIG. 5B, as the retry limit increases, more retransmissions may be deployed in case of severe channel damage. Combining Figures 5A and 5B, power consumption is shown in Figure 5C. Power consumption is scaled by _sf c, as shown in equation (18). The optimal point OP here occurs at N _lim = 1, i.e., increasing the transmit energy E _t per bit for a high p _L is always more efficient than increasing the number of transmissions. Comparing the power consumption at the optimal point with the power consumption when N _lim = 10, about 50% power savings can be obtained.

In Figures 6A-6C, the case of p _L = 0.01% is shown in Figures 5A-5C indicating higher received video quality for the same channel condition. Comparing FIGS. 6A-C and 5A-C, it can be seen that for higher quality, a larger retry limit is needed to achieve higher error correction performance. In particular, as shown in FIG. 6C, when p _L = 0.01%, the optimal point is the retransmission limit. Occurs when

Minimize power consumption over a range of distances

In this paragraph, we analyze the case where the receiving terminal moves (for example, a distance of 10m to 20m, simulating in a home environment). For each distance, the optimal pair N _lim , E _t is calculated. The results are summarized in FIG. Where E _t is Is normalized, where d ₀ = 10m. If the distance is greater, it has been found that higher retry limits are desirable. The power consumption for N _lim = 10 is also shown in FIG. 7. Comparing the two curves, the algorithm of the present invention outperforms the technique in which N _lim is set to a large value. If N _lim is set to a small value, for example N _lim = 1, it repeats the optimal curve for a small distance up to d = 16 in this simulation, but cannot adapt to a long distance. In other words, the quality can be maintained at a desired level. There is some inconsistency for the optimal curve and the variation for the N _lim = 10 technique. This is due to the fact that for discrete N _lim and E _t sets, the resulting PSNR is in fact not constant, but is always larger than expected.

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The 802.11 MAC / PHY standard allows the device to change the transmit energy level and retry limits in progress. Increasing both the retry limit N _lim at the MAC layer and the transmit energy level E _t at the physical layer PHY provides higher error protection for the transmitted data. However, in order to reach the same video quality at the receiver, they operate differently in terms of power consumption. The present invention determines the optimal pair N _lim , E _t to minimize power consumption.

Flow chart 100 and system diagram 200 illustrating an implementation of the present invention are provided in FIGS. 8 and 9, respectively. This implementation provides a "power manager 102" whose operation may be distributed over a wireless network between the base station B and one or more portable terminals TER.

In step S1, the adaptation rules for the discrete quality requirement set, channel 114 conditions and video sequence characteristics (e.g., the relationship between the PSNR and the rate of the FGS encoder 112) are precomputed and the base station B Stored as a lookup table. The optimal working pair N _lim , E _t is provided to the lookup table 104 for each data set.

In step S2, during communication of the scalable video sequence, QoS requirements, channel conditions and video sequence characteristics are detected and reported to the power manager 102. Based on this criterion, the power manager 102 determines the optimal operation pair N _lim , E _t by accessing the pre-calculated lookup table 104. N _lim from the best working pair is provided to the MAC layer 116, while E _t from the best working pair is provided to the PHY layer 118. In step S3 these operating points are frequently updated to follow the time varying and application specific characteristics of the wireless channel 114.

The invention can be implemented in hardware, software, or a combination of hardware and software. Any kind of computer / server system (s), or other apparatus that performs the methods described herein, is suitable for the practice of the present invention. A typical combination of hardware and software may be a general purpose computer system having a computer program that, when loaded and executed, performs each of the methods described herein. Alternatively, a particular purpose computer may be used, including dedicated hardware to perform one or more functional tasks of the present invention. The invention includes all the individual features that enable the implementation of the methods described herein, and can also be implemented as a computer program capable of performing these methods when loaded into a computer system. A computer program, software program, program or software in this context may cause a system with information processing capability to directly (a) convert another function, into another language, code or annotation, and / or (b) a different material form. Any expression, by any language, code, or comment, on a set of instructions that is to be performed after one or both of the regenerations of.

An example of computer system 300 is shown in FIG. 10. Computer system 300 generally includes central processing unit (CPU) 302, memory 304, input / output (I / O) interface 306, bus 308, external device 310, and database 312. ). User 314 may interact with computer system 300 (eg, to create lookup table 104 (FIG. 9)).

Computer 300 may include any general purpose or special purpose system that is designed to drive the operation of particular hardware and includes standard operating system software that is compatible with other system components and I / O controllers. CPU 302 may include a single processing unit, multiple processing units capable of parallel operation, or may be distributed across one or more processing units at one or more locations, for example, on a client and a server. Memory 304 includes any known kind of data storage and / or transmission media, including magnetic media, optical media, random access memory (RAM), and the like. In addition, similar to CPU 302, memory 304 may reside in one physical location, including one or more types of data stores, and may be distributed across multiple physical systems of various shapes.

I / O interface 306 may include any known system for exchanging information with one or more external devices 310. External device 310 may include any known type of input / output device capable of communicating with I / O interface 306 without additional devices. The bus 308 provides any communication link between the respective components within the computer 300 and may include any known transmission link that includes the same, electrical, optical, wireless, and the like. Other known components can also be integrated into the computer 300.

Database 312 provides a repository for information needed to carry out the invention. For example, lookup table 104 (FIG. 9) is stored in database 312. Database 312 may include one or more storage devices, such as a magnetic disk drive or an optical disk drive. In addition, the database 312 may include data distributed over a network, such as a LAN, WAN, or the Internet.

Power manager 320 in accordance with the present invention is shown as being stored in memory 304 as computer program code. Power manager 320 accesses an information system 322 that determines / receives " transmission characteristics " such as QoS requirements, channel conditions, video sequence characteristics, and the like, and accesses a pre-computed lookup table stored in database 312 each time ( An optimization system 324 that determines the optimal pair of operations N _lim , E _t for T). N _lim and E _t are provided sequentially to the MAC and PHY layers 116, 118 (FIG. 9) via the I / O interface 306.

The foregoing description of various aspects of the invention has been presented for purposes of illustration and description. The form disclosed is not intended to be exhaustive or to limit the form of the present invention, and obviously, many modifications and variations are possible. Such modifications and variations apparent to those skilled in the art are included within the scope of the invention as defined in the appended claims.

Claims (18)

What is claimed is: 1. A method for power-efficiently transmitting scalable video over a wireless network, the method comprising: Generating a lookup table comprising an optimal pair (N _lim , E _t ) for a plurality of different transmission characteristic sets, where N _lim is a retry limit and E _t is transmission energy per bit; Determining a set of transmission characteristics for a scalable video sequence to be transmitted over the wireless network; Accessing the lookup table to obtain an optimal pair N _lim , E _t corresponding to the determined set of transmission characteristics; Transmitting the sequence of scalable video over the wireless network using the accessed optimal pair N _lim , E _t . How to include. The method of claim 1, Each transmission characteristic set comprises at least one of a quality of service requirement, a channel condition, and a video sequence characteristic. The method of claim 1, Encoding the sequence of scalable video using a Fine-Granular-Scalable (GFS) encoder. The method of claim 1, The sequence of scalable video is encoded using a base layer encoder and an enhancement layer encoder. The method of claim 1, Providing N _lim to a medium access control (MAC) layer of the wireless network; Providing E _t at the physical layer of the wireless network How to include more. The method of claim 1, Repeating the determining, accessing and transmitting steps to follow the time varying characteristics of the wireless network. A system for power efficient transmission of scalable video over a wireless network, the system comprising: A lookup table containing an optimal pair (N _lim , E _t ) for a plurality of different transmission characteristic sets, where N _lim is a retry limit and E _t is a transmit energy per bit; A system for determining a set of transmission characteristics for a scalable video sequence to be transmitted over the wireless network and accessing a lookup table to obtain an optimal pair (N _lim , E _t ) corresponding to the determined transmission characteristic set; A system for transmitting the sequence of scalable video over the wireless network using the accessed optimal pair N _lim , E _t . System comprising. The method of claim 7, wherein Each transmit characteristic set comprises at least one of a quality of service requirement, channel condition, and video sequence characteristic. The method of claim 7, wherein And a FGS encoder for encoding the sequence of scalable video. The method of claim 7, wherein And a base layer encoder and an enhancement layer encoder that encode the sequence of scalable video. The method of claim 7, wherein The wireless network includes a medium access control (MAC) layer, N _lim is provided to the MAC layer of the wireless network, and E _t is provided to the physical layer of the wireless network. The method of claim 7, wherein The determination system updates the transmission characteristic set after a predetermined interval, accesses the lookup table to obtain an updated optimal pair N _lim , E _t corresponding to the updated determined transmission characteristic set, The transmission system transmits the sequence of scalable video over the wireless network using the updated optimal pair N _lim , E _t . system. A program product stored on a recordable medium for providing power efficient transmission of scalable video over a wireless network, the program product comprising: Program code for determining a set of transmission characteristics for a sequence of scalable video to be transmitted over the wireless network; Corresponding to the determined set of transmit characteristics by accessing a lookup table that includes an optimal pair (N _lim , E _t ) for a plurality of different transmit characteristic sets, where N _lim is a retry limit and E _t is transmit energy per bit. _Obtain the optimal pair N _lim , E _t , wherein the sequence of scalable video is transmitted over the wireless network using the accessed optimal pair N _lim , E _t . Program product comprising a. The method of claim 13, Each transmit characteristic set comprises at least one of a quality of service requirement, a channel condition, and a video sequence characteristic. The method of claim 13, And the sequence of scalable video is encoded using an FGS encoder. The method of claim 13, And the sequence of scalable video is encoded using a base layer encoder and an enhancement layer encoder. The method of claim 13, Program code for providing N _lim to the MAC layer of the wireless network, Program code for providing E _t to the physical layer of the wireless network Program product comprising more. The method of claim 13, Program code for updating the set of transmission characteristics after a predetermined interval and accessing the lookup table to obtain an updated optimal pair N _lim , E _t corresponding to the updated determined transmission characteristic set; The sequence of scalable video is transmitted over the wireless network using the updated optimal pair N _lim , E _t . Program product.